The objective of this study is to evaluate the feasibility of H2 (15)O PET for the measurement of synovial blood flow in patients with RA. In 10 RA patients with active knee synovitis, clinical and radiological (H2 (15)O PET and dynamic MRI) evaluation will be performed. The feasibility of H2 (15)O PET will be evaluated by its reproducibility. We will also evaluate the correlation of dynamic MRI with synovial blood flow measurement by H2 (15)O PET.